Therapeutic systems for the transdermal administration of medicaments supply one or more active substances at a predetermined rate and in continuous manner over a fixed period to a given application point on the skin.
These systems are therapeutic precision instruments ensuring a continuous active substance release.
Such therapeutic systems can have both a topical and a systemic action and the large number of active substances which can be applied in this way and their different chemical, physical and pharmacological characteristics make ever new demands on the production of such systems.
Conventionally these transdermal systems have at least one active substance reservoir, where the active substance is present in solid, liquid or disperse molecular form and an adhesion layer through which the system is closely connected with the skin and through which active substance transfer takes place, a control membrane and protective/covering layers which are substantially impermeable for the active substance.
The known systems are difficult to manufacture and have a complicated structure.
One problem of conventional systems is that of being able to process readily volatile active substances, because the evaporation of the active substance is difficult to control during production.
Thermally sensitive active substances can only be used to a limited extent in the system in the case of matrices or therapeutic systems which have to be thermally treated and which are produced with heat treatment stages.
Attempts have already been made to introduce pure active substance in fine-crystalline form into a pressure sensitive adhesive polymer, so that the finely divided, fine-crystalline active substance dissolves with time as depot crystals in the adhesive matrix layer (DE-OS 35 00 508=U.S. Pat. No. 4,719,226). This process is not suitable for volatile and thermally sensitive active substances, because it includes thermal treatment stages.
Another attempt to increase the capacity of such therapeutic systems comprises embedding in a pressure sensitive adhesive layer of such a system active substance depots in the form of microcapsules, which are surrounded by a control membrane (see U.S. Pat. Nos. 3,598,123 and 3,731,683). The production of such control membrane-surrounded microcapsules is extremely complicated and expensive and cannot be performed for many active substances. The mixing of the active substance-containing microcapsules under a reservoir material constitutes a further difficult process stage, during which the microcapsules can easily be damaged or destroyed, which can lead to an unsatisfactory constancy of the active substance content in the finished therapeutic system. The process of U.S. Pat. No. 3,598,123 is difficult to perform for liquid active substances, particularly if the liquid substance is present in readily volatile form.
German patent 3 424 837 discloses a depot plaster, which can be used for liquid materials and has a covering film, a liquid active substance in an outwardly bulging a region of the covering film and a control membrane covering the active substance and permeable for the latter. Between the covering film and the control membrane is provided an active substance distribution device, namely a non-woven fabric, which uniformly distributes the active substance liquid on the control membrane and which is effective over a large surface area. In the case of the depot plaster of German patent 3 424 837 the covering film and the control membrane are welded together in their outer regions in order to prevent an outward flow of the liquid active substance.
However, the known depot plaster is disadvantageous in that the liquid therein flows freely and can easily run out if the adhesive or welded edges are damaged and also requires an expensive control membrane, which must be provided in addition to the active substance distribution device in order to kinetically control the delivery of the active substance.
The problem of the present invention is consequently to provide a novel therapeutic system with active substance depot for the administration of the active substance, which can be manufactured less expensively and more reliably than the prior art systems, which is also suitable for processing volatile and/or thermally unstable components.